1. Field of the Invention
The present invention generally relates to hydrostatic transaxles and, more particularly, is concerned with a control arrangement for the pump track ring.
2. Description of the Prior Art
Hydrostatic transmissions transmit rotary mechanical motion, typically from an internal combustion engine, to fluid motion, then back to rotary mechanical motion to rotate a drive axle in order to drive a vehicle such as a lawn and garden tractor or riding mower. The hydrostatic transmission regulates or controls the output rotary mechanical motion such that varying output speeds in the forward and reverse directions are possible with a single speed input rotary mechanical motion. Within a hydrostatic transmission of the radial piston type, a pump and motor each having a cylinder unit that rotates on a fixed pintle with pistons positioned within the cylinders and attached to slippers mounted in an expander band so that as the cylinder unit rotates, the slippers engage the surrounding eccentric annular track ring of the pump and motor. The pistons of the pump create a pressurized fluid flow that drives the motor pistons which rotate an output shaft. The transmission ratio is therefore directly proportional to the eccentricity of the track ring of the pump relative to the fixed pintle.
The eccentricity of the pump track ring must therefore be variable and this is accomplished by pivoting the track ring around an axis located at one end of the track ring, the axis generally being a pivot pin. In addition, a control mechanism adapted to swing or pivot the track ring around the pivot axis must also be provided so that an operator can change the eccentricity of the track ring.
Generally the track ring pivots around a pin or rod extending at least through the track ring on the inboard side, or proximal side relative to the gearing, of the transaxle casing and is held by the clamping force exerted by the two transmission casing halves that are bolted together. It is necessary, however, for the pivot pin to be mounted as securely as possible since the pivot pin bears a large amount of load. This is because of the pressure exerted on the pump track ring by the hydrostatic pressure within the pump and the torsional forces created by the pivotal movement of the track ring. This type of hydrostatic transmission is shown in U.S. Pat. No. 4,979,583, entitled VARIABLE SPEED TRANSAXLE, issued Dec. 25, 1990, and pending application Ser. No. 07/535,462, entitled VARIABLE SPEED TRANSAXLE, filed Jun. 8, 1990, both of which are specifically incorporated herein by reference.
The control rod mechanism for the pump track ring has heretofore been located on the outboard side, or distal side relative to the gearing, of the transaxle casing. This is disadvantageous in that the transmission casing must be extended in the forward direction as it is necessary to accommodate the control rod mechanism, which can cause interference with or the restriction of the area in which the mower blade deck is located on a conventional riding mower, in which the transmission is installed. It is thus desirable to maintain the front dimension of the transmission casing as small as possible so as to allow sufficient room for the mower deck to be raised.